Amplifier gas tube



Sept. 23, 1952 w. M. WEBSTER. JR., ET AL 2,61

AMPLIFIER GAS TUBE Filed March 28, 1951 2 SHEETS--SHEE'I l .5 VOL 75wvzA/roks MAL/4M M Wzasmi, J/a 6.60565 W fi/r/A/ Me Patented Sept. 23,1952 AMPLIFIER GAS TUBE William Merle Webster, Jr., Princeton, andGeorge W. Bain, Jr., New Brunswick, N. 3., assignors to RadioCorporation of America,"a corpora-v tion of Delaware Application March28, 1951, Serial No. 217,912

This invention relates to improvements in gas tubes which are suitablefor use as amplifiers in that they have continuous grid control. Moreparticularly it relates to improvements in amplifier gas tubes, of aparticular kind having very high values of transconductanceand of anodecurrent and extremely low values of output impedance, which aredescribed in co-pending application, Serial No. 185,745 which was filedon September 20, 1950, and was assigned to the same assignee as thepresent application and to novel circuits utilizing the improved tubes.

Tubes of this kind, which are referred to as "Plasmatrons, have offeredvery great advantages over the tubes which preceded them. However, someof them also have certain limitations. Before these limitations can beproperly understood, it is necessary first to review the basic operatingprinciples of the plasmatron. In this kind of tube there are separatedischarge paths for the load current and the ionizing current.Theenergizing potential required for drawing the former, the loadcurrent; from a main cathode to a main anode, is set at well below thevalue required to produce ionization. However, a higher potential. isused to produce a separate ionizing, or auxiliary discharge. It ionizesthe tubes gaseous filling 1 converting it into a conductive plasmaconsisting of positive ions and detached, free, negative electrons. Theplasma (1) surrounds the main cathode and (2) fills the load currentdischarge path whereby (1) it neutralizes the. electron space chargesurrounding the main'cathode and causes a very great increase inemission, and (2) it efiectively acts as a low impedance conductorconnected between the main cathode and the main anode. Because of theseefiects the load current is very large, e. g., ampere, despite the verylow main anode potential, e. g., 5 volts. In'addition, the overallefficiency is very high since the great multiplication of load currentwhich results from the use of an auxiliary discharge, can be attainedeven though the current-of that discharge is itself of very smallmagnitude.

In prior art plasmatrons the presence of the plasma influences only thedirect current component of the load current. This is because theauxiliary discharge is energized with a steady direct potential wherebythe density of the plasma is substantially constant as initiallygenerated and because the application of a signal to the plasmatroncontrol grid is its location athwart the path of the low-Velocity loadcurrent is incapable of influencing the plasmadensity. In

16 Claims. (01. 315-468) fact, as will be more clearly understood fromwhat follows, it is because a control grid located between the maincathode and main anode is incapable of influencing the generation ofplasma that it is able to retain control of the load current even thoughit is entirely immersed in the plasma. Incidentally, it should be notedthat the control which it does have is not based oncooperation with thefield of an unneutralized, negative, cathode space-charge as in the caseof the control grid of a conventional hard tube. Instead its control isbased on the following: the grid, when negatively biased, is surroundedby a positive-ion sheath which the input signal varies in thickness.These variations in turn vary the cross-sectional areas, and theconductivity, of

columns of conductive plasma which extend thru the grid openings betweenthe main cathode and the main anode.

The attainment and retention of this type of load current control in agas tube marked a great advance in the art. In most other kinds of gastubes, e. g., thyratrons, in which the grid is unable to retain control,if indeed it ever has any control at all beyond that of firing the tube,continuous control is impossible because the ionizing discharge occursover the very same path as the load current discharge and thereforeextends through the grid opening(s). the. grid is deprived of anycontinuing ability to control the anode current once the gaseous fillingof the tube becomes ionized. The mech'anics'of how the thyratron grid isdeprivedof control is that when located in the ionizing path it hasindirect effects on load current, as a result .of at! fecting the plasmadensity, which are opposite to, and exactly offset, its direct eiiects.In view of the foregoing description of plasmatrons .it is apparent thatthese mechanics do not applytoit and that this is why its grid does notlose control.

Three limitations encountered inplasmatron tubes are: that theirgrid-voltage versus plate- I current characteristics are often somewhatnonlinear; that they have lower input impedances than are desirable forcertain purposes; and that their attainable transconductances are ratherlimited due to the fact that their control grids must be capable ofdirectly controlling very large magnitude currents.

It is the object of the present invention to devise a gas amplifier tubeof the kind in question with a grid-voltage versus plate-currentcharacteristic of improved linearity.

It is a further object of the present invention As a result 3 to providesuch tubes, and circuits for them, which have increased values of inputimpedance.

It is a further object of the present invention to provide gas tubeamplifiers of the kind in question, and circuits for them, withincreased values of transconductance.

These objects have been obtained by making certain modications in thetubes and by devising novel amplifier circuits for employing them tomaximum advantage.

According to the principal tube modifications of the present inventionthe load-current control grid, which is normally included between themain anode and the main cathode, is replaced or supplemented by aconstriction-modulating electrode, which is placed in the path of theauxiliary discharge. The purpose in so doing is to indirectly effectcontrol of the load current by varying the density of the plasma withoutat the same time directly controlling it in an opposit and compensatingmanner. Since the rate at which the-density of the plasma can diminishis a function of deionization time, and therefore is limited, this typeof control results in lowering the upper limit of the frequency range ofoperation. However, more linear e. g./I. P. characteristics result fromthis kind of control while at the same time the obtainable upperfrequency limit is still high enough for many applications such as foraudio use and in servo systems. In addition since direct control of arelatively small ionization current results in indirect control of aVery large load current, the obtainable transconductance benefits from amultiplication efiect analogous to that obtained in secondary emissionelectron multiplier tubes. Therefore, extremely large transconductance,such as the order of one or mor mhos have been obtained. Furthermore,for reasons which will be explained in detail below, the input impedanceto the constriction electrode can be very high when it is properlybiased. According to the principal features of the present invention:(1) a particular energizing circuit is used for the auxiliary dischargein combination with the use of a constriction electrode in the tubewhereby it is possible for very small changes in the input signalvoltage to cause surprisingly large changes in the density of theplasma; and (2) the constriction electrode is biased at an optimum pointin a particular negative voltage range wherein the electron currentwhich it collects, due to the fact that thermal velocities of electronsemitted by the auxiliary cathode exceed this negative bias, is ofiset byan opposite current flow of electrons which it gives up to the positiveion sheath surrounding this electrode. Incidentally the electronsconstituting this latter current are continuously neutralizing some ofthe ions in the sheath but as fast as these diffuse away new replacementions are drawn to the electrode from the plasma to replenish the sheath.

The result of the optimum bias for the constricting electrode is thatthe two oppositely moving currents are dynamically balanced whereby theefiective input impedance of the electrode, for small input signals,would be infinitely large except for its capitance to ground and to theother electrodes.

Since the constriction-modulating electrode can be made of very smallphysical size to keep down capacitance values, the actual total inputimpedance is very large.

In the drawing:

Figure 1 is a longitudinal sectional view of an illustrative embodimentof a tube according to 4 the present invention. The section is takenalong lines I-l of Figure 2 and in a plane which is parallel to the axesof the cylindrical cathodes;

Figure 2 represents another longitudinal sectional view of thisembodiment, this section being taken along the line 2-2 of Figure 1 in aplane perpendicular-to that of the section of Figure 1;

Figure 3 is a schematic circuit diagram of an amplifier system using thetube shown in Figures 1 and 2;

Figure 4 shows the anode-current versus constriction-electrode-voltageof a tube as in Figures l and 2;

Figure 5 represents a longitudinal sectional view of a modification ofthe tube shown in Figure 1, the section being taken like that of Figure2;

Figure 6 is a schematic circuit diagram of a mixer system using the tubeof Figure 5; and

Figure 7 shows the characteristic obtained by plotting, in rectangularcoordinates, the constriction-modulating-electrode current against thevoltage between this electrode and the auxiliary cathode.

The gas tube [0 shown in Figures 1 and 2 comprises a gas tight envelopeII which, in this illustrative embodiment, is of substantiallyrectangular shape. The load current in this tube originates from a mainindirectly heated cathode I2 and is received at a main anode 13. Anauxiliary indirectly heated cathode 14 serves as a source of electronsfor the ionizing discharge. A slotted shield electrode I5 is placedaround the auxiliary cathode l4. It forms the auxiliary discharge into astream which has a narrow constriction Where it emerges from the shieldand is directed at the main. cathode [2. The presence of a constrictionin the ionizing stream reduces its current thereby raising the impedanceof the auxiliary discharge by making it work hard. In other words byusing the shield electrode a higher are. drop is required to start andto sustain the auxiliary discharge. The are drop automatically adjustsitself so that a suflicient number of ionizing collisions for aself-sustained discharge will occur, despite the small magnitude of theionizing current, because of the increased average velocity of theelectrons constituting this current. In this arrangement these electronsattain ionizing velocities after they have traversed only a fraction of.the distance between the slot I! in the shield electrode l 5 and thenearest electrode which is polarized to attract them, i. e., the maincathode, therefore, since they are capable of, having ionizingcollisions in traversing all of the remainder of this distance, theefliciency of the auxiliary discharge as a means for generating plasmais greatly increased. If the shield 15 were not employed, the auxiliarycurrent would be very large and the arc drop between the auxiliarycathode l4 and the group of electrodes servin as a composite auxiliaryanode, the main cathode l2 and the main anode [3 herein, would be onlyslightly above the ionizing potential for the gaseous filling of thetube. Because of this the ionizing efliciency of the auxiliarydischarge, and therefore the efiiciency of the entire tube, would bepoor. Hence the shield is rarely omitted in plasmatron practice.

According to the present invention the shield electrode is always usedand according to its principal feature a constriction-modulatingelectrode 1-6 is used in combination with it to cooperate with its slot11. The modulating electrode 16 in the example shown herein comprises apair of parallel wires or rods [8 connected together at their tops andbottoms by conductive spacers H! to provide a unipotential electrodestructure having a slot 2d substantially coinciding with the slot IT.The electrode I6 is supported between the top and bottom of theenvelope. H by rods 2! and 22, the former of which is sealed partwaythrough the bottom of the envelope to act merely as a support elementwhereas the latter extends through the top of the envelope to serve as aterminal pin.

When the electrode 16 is negatively biased the direct effect of applyinga signal. voltage to it is to vary the thickness of a positive ionsheath which surrounds it and hence the indirect effect is toeifectively vary the size of slot 28.

Each of the two cathodes, i. e., the main cathode l2 and the auxiliarycathode i4 is supported on individual rods 23 and 24 each of whichextends through the top of the envelope I I to serve as a terminal pinfor polarizing the cathode and for conducting an electrical current toand/or from one end of its internal filamentary heater. In additionthese cathodes are provided with terminal pins 25 and 26, also sealedthrough the top of envelope ll, each of which serves for conducting thesame current from and/or to the other end of one of the heaters. Themain anode I3 is supported by two rods 2'! which are fused part waythrough the envelope H and another rod 28 which is fused entirelythrough it whereby it may be used as an external terminal. The shield issupported on a rod 29 which extends,

like the rods 23. 24, and 28, through the envelope so that it may alsoserve as a terminal pin.

Tube It may be processed in any number of ways well known in the art toprovide a gaseous filling within its envelope prior to sealing off. Anysuitable gas or mixture of gases may be utilized. The gas pressure forany particular embodiment will be in accordance with its specificelectrode geometry and spacings and must be such to favor the formationof a self sustaining ionizing discharge. A number of plasmatron tubeshave been found to operate satisfactorily with a filling of helium of apressure of approximately 750 microns. However, as is well known othergases and other pressures may be used, c. g. pressures which lie withinthe range of between approximately 100 microns and several millimetersof mercury.

Figure 3 shows a circuit which is suitable for using the gas tube shownin Figures 1 and 2. In this figure three direct potentials sources areshown, a low-potential load-current-energizing source 30; a highpotential auxiliary-dischargeenergizing source 3!; and a biasing-source32. The source 30 is used for maintaining the small non-ionizingpotential difference between the main cathode l2 and the main anode l3.It may consist of a number of series-parallel connected 1 /2 volts drycells, a storage battery, or any similar fairly-high-current,low-voltage source. This is because of the fact that although it mustnot provide much voltage it must be capable of delivering to an outputcircuit 33 a very substantial signal-bearing current, i. e., a currentwhich may easily have an average value of the order of one to severalamperes. Conversely the source 3! should be capable of providing arelatively much higher potential but need not be capable of providing aparticularly large continuous average current. This is because it servesto energize the eflicient, low-current ionizing-discharge. Finally thesource 32 is only called upon to provide a low voltage and that withlittle current since it merely provides a small negative bias betweenthe constriction-modulating electrode [6 and the auxiliary cathode M. Inthe circuit of Figure 3 the main cathode is at the direct potential ofground and the main anode is nearly so since the source 3!] establishesonly a small voltage difference between these elements. As toalternating currents they are both grounded since the source 30 may beassumed to have a low internal alternating current impedance whetherv itis a battery with Or without a by-pass condenser or an equivalent lowvoltage power supply. Connected in series with the source 30, the maincathode l2 and the main anode [3 there is the primary of an outputtransformer 33 for coupling the varying component of the output currentof tube It to a utilization device (not shown) such as a loud speaker.In this arrangement the main anode and main cathode serve togetheras acomposite anode for attracting and receiving electrons from theauxiliary cathode M to establish the auxiliary discharge. Accordingly,the source 3| is connected between the auxiliary cathode l4 and ground,i. e., the voltage reference for the composite anode, in such polaritythat the auxiliary cathode is sufiiciently negative with respect theretoto produce the required ionizing discharge.

As is well known, it is necessary to use a current limiting means inseries with a, gaseous dis charge to prevent such a great current flowas to damage the gas tube in which it occurs and/ or the circuit orsource feeding it. The means usually employed is a series resistor ofrelatively very large value with respect to the internal impedance ofthe gas tube. In such an arrangement small current changes can producemarked changes in the cathode-to-anode voltage impressed across the tubewhich therefore tendsto act as a constant current device. According tothe present invention, the series resistor employed between the source3| and the auxiliary cathode M has a relatively very small value withrespect to the impedance of the auxiliary discharge, e. g., the value ofthe resistor 34 shown herein may be as little as 75 to 100 ohms. Twofeatures of the present invention are that it is possible to. use asmall limiting resistor because of the arrangement'of tube l0 and thatit is advantageous to do so as will be explained. The reason why it iseven possible to use such a small limiting resistor without drawingexcessive current is that the constriction employed in the ionizingdischarge herein so greatly increases the impedance'of the auxiliarydischarge that it alone almost suffices adequately to limit its owncurrent. The reason why it is advantageous is that by choosing a source3! which provides a voltage only a little larger than that required tosustain a, going arc in the tube Iii, while at the same time choosing aresistor 34 which is small with respectto the arc impedance a new typeof load current control becomes possible.

. Under these conditions most of the total IR drop it will be possibleto effect large changes in its auxiliary discharge current by varyingthe arc impedance. According to the present invention, the arc impedanceis controlled by modulating the constriction in the auxiliary discharge,This is accomplished by applying a signal voltage to the constrictionmodulating electrode [6 to dynamically vary its negativedirect-potential bias. This markedly changes the arc impedance byeffectively changing the width of the slot 26, through varying thesheath thickness as explained above, thereby changing both the voltagedrop across the arc and the current through it. Though each change inthe voltage drop across the arc has an opposite effect on the plasmadensity to that ofthe accompanying change in the current through thearc, the magnitude of the latter change is by far the greater. Forexample, as a negative signal-voltage increment is added to the negativebias of the constriction-modulating electrode, the plasma sheath aboutit will thicken; the arm impedance will increase; the arc drop willincrease; and the arc current will decrease. The increase in the arcdrop will cause the ionizing electrons to move with higher velocities sothat on the average each of them will have more ionizing collisions. Atthe same time, however, the reduced auxiliary current will tend tolessen the number of ionizing collisions, and it is this effect whichwill predominate. Therefore, the overall effect of such a signalincrement will be to cause a. reduction in the plasma density and in theload current which it supports. A characteristic curve obtained intesting a tube of the type shown in Figures 1 and 2 is shown in Figure4. It indicates that the load current decreased in a fairly linearmanner from about (1) ampere down to substantially zero current as theconstrictionmodulating electrode bias was varied from zero volts to 1volt. V

In the circuit of Figure 3 the basing source is shown to be connectedbetween the auxiliary cathode l4 and the constriction modulatingelectrode I6 by a circuit including the secondary of an inputtransformer 35. The dotted line 39 is intended to show the shieldelectrode [5 may be biased from the same source as the constrictionmodulating electrode or that it simply may be left floating. When theshield electrode l5 is left floating it will become self biasedbycollecting thermal electrons from the auxiliary cathode l4 until it hascharged in a negative direction to a point sufficiently below thepotential thereof to cease to attract electrons therefrom. Where theshield electrode is biased by the use of an external source ofpotential, it is not necessary to bias it at the same potential as theconstriction modulating electrode.

Figure 5 is a modification of the tube shown in Figs. 1 and 2 where twoload current modulating means are employed; (1) a constrictionmodulating electrode of the kind described in detail above, and a loadcurrent control grid 36 which may be of the kind(s) described in theabove-mentioned copending application, Serial No. 185,745. The result isa dual input tube which may be useful in avariety of applications suchas a very low output impedance mixer.

Figure 6 shows a schematic circuit diagram for a mixer circuit which issuitable for using the tube of Figure 5. It is substantially the samecircuit as that shown in Figure 3 except that the control grid 36 isrepresented in this circuit along with a second signal input circuitwhich circuit comprises a transformer 37 and a biasing source 38connected in a manner which is selfexplanatory as shown.

While it is preferable for the limiting resistor 34 to have a valuewhich is but a fraction of the impedance of the ionizing discharge, itmay have difierent relative values with the following results: (1) itmay have a value which up to a certain point approaches or even exceedsthat of the arc impedance provided it is employed for applications inwhich lower values of transconductance are adequate. Up to that certainpoint increases in the value of resistor 34 will result in progressivelylower transconductances since the disparity between the change in onedirection in the plasma density which is caused by a given change in theionizing current and the opposite change in plasma density caused by theaccompanying change in the arc drop will become progressively smaller asthe value of resistor 34 is increased. (2,) It may have even highervalues in a range in which the disparity between the two effects onplasma density which are mentioned above will have diminished to 0 andthen reappeared in the reversed sense. This is a range in which thechanges in plasma density produced by changes in the arc drop will havebecome predominate over the others. In this range a characteristic curveplotted in rectangular coordinates as in Fig. 4 will slope in the otherdirection, and the transconductances represented by it will be negative.(3) It may have a value near zero or be omitted entirely, providing thearc is made to work so hard, for example, by the use of an extremelynarrow constriction, that its impedance alone is capable of'sufiiciently limiting the current. Of course, the resistor 34 may inany instance be eliminated if the source 3! has enough internalimpedance to provide the limiting effect or if it be replaced by somesuitable equivalent such as the cathode-anode circuit of a dischargedevice.

What is claimed is:

1. A gas amplifier tube comprising: a sealed envelope containing agaseous filling; means for carrying a load current through the tubecomprising a main cathode and a main anode in cooperative spacedrelationship; means including an auxiliary cathode for producing anionizing discharge to provide a conductive plasma between said maincathode and anode; means adjacent said auxiliary cathode for forming theionizing discharge into a directed stream having a constricted portionof smaller cross-sectional area than the rest; and means forconstrictionmodulating said stream to thereby modulate the plasmadensity.-

2. A gas tube as in claim 1 in which the means for forming the ionizingdischarge into a stream comprises a slotted shield between the auxiliarycathode and said first-mentioned means with its slot in alignment withthe path along which said stream is to be directed.

3. A gas tube as in claim 2 in which said constriction-modulating meanscomprises a slotted electrode having a small total area as compared tosaid shield and positioned with its slot in substantial registry withthat of the shield.

4. A gas tube as in claim 1 and including a control grid between saidmain cathode and main anode.

5. A gas tube comprising: a sealed envelope containing a gaseousfilling; a group of load circuit electrodes within the envelope andincluding at least one main cathode and a main anode having,respectively, electron-emitting and electron receiving surfaces whichdefine opposite ends of a predetermined load current path; meansincluding an auxiliary cathode for producing an ionizing discharge ofelectrons along a path within said envelope which does not coincide forall of its length with all of said load current path but includes aportion in such close proximity thereto, for example, coincidentaltherewith or crosswise or adjacent thereto, that said ionizing dischargebathes the main cathode and extends over all of said current path; aslotted shield adjacent said auxiliary cathode for forming the ionizingelectron current into a directed stream including a constricted portionof smaller cross-sectional area than the rest; a slotted electrodepositioned with its slot in substantial registry with that of theshield; and terminal means for applying a signal to said slottedelectrode to constriction-modulate said stream.

6. A gas tube as in claim 5 and including a control grid positionedathwart said load current path.

'7. A gas amplifier tube comprising: a sealed envelope containing agaseous filling; means for carrying a load current thru the tubecomprising a main cathode and a main anode in cooperative spacedrelationship; means including an auxiliary cathode for producing anionizing discharge to provide a conductive plasma between said maincathode and main anode; means adjacent said auxiliary cathode forforming the ionizing discharge into a directed stream including aconstricted portion; means for constriction-modulating said stream tothereby modulate the plasma density; and a control grid between saidmain cathode and said main anode.

8. A gas tube comprising a sealed envelope containing, a gaseousfilling, within the envelope an auxiliary cathode, a main cathode, andan anode positioned in said order, a slotted shield between saidauxiliary cathode and said main cathode with its slot in alignment witha straight line therebetween, and a slotted electrode having a smalltotal area as compared to that of the shield and positioned with itsslot in substantial registry with that of the shield.

9. A tube as in claim 8 and further comprising a control grid betweensaid main cathode and said main anode.

10. A tube as in claim 8 in which the main cathode is cylindrical and isadapted to emit radially and the main anode surrounds it in most radialdirections except those towards said auxiliary cathode.

11. An amplifier circuit comprising a tube as in claim 1 and a source ofpotential connected between said auxiliary and main cathodes, saidsource providing a potential somewhat greater, but not very muchgreater, than the sustaining potential of the ionizing discharge, andsaid source being connected therebetween over a resistor having asmaller value than the efiective resistance of the ionizing discharge,whereby during the operation-of the tube the average voltage dropbetween the auxiliary cathode and the main cathode is greater than thatacross said resistor.

12. An amplifier circuit comprising a gas tube as in claim 5, a sourceof potential connected between said auxiliary and main cathodes, saidsource providing a potential greater than the sustaining potential ofthe ionizing discharge, said source being connected therebetween over aresistor, and a source of potential biasing said slotted electrode at anegative potential with respect to said auxiliary cathode.

13. An amplifier circuit as in claim 12 whereby in the operation of saidcircuit a sheath of positive ions will surround said slotted electrodeand in which said negative biasing potential has a predetermined valueat which the current of electrons which the slotted electrode receivesfrom the auxiliary cathode is in substantial dynamic balance with thecurrent of electrons which it gives up to said sheath.

14. An amplifier circuit as in claim 13 in which the potential providedby said first-mentioned source is somewhat greater than the sustainingpotential of the ionizing discharge but not much greater and saidresistor has a value smaller than the effective resistance of theionizing discharge.

15. An amplifier circuit as in claim 14 in which said resistor has avalue which is but a small fraction of said effective resistance.

16. An amplifier circuit comprising a tube as in claim 5 and a source ofpotential biasing said electrode at a negative potential with respect tosaid auxiliary cathode whereby in the operation of said circuit a sheathof positive ions will surround said slotted electrodes, said negativepotential having a predetermined value at which the current of electronswhich the slotted electrode receives is in substantial dynamic balancewith the current of electrons which it gives up to said sheath.

WILLIAM MERLE WEBSTER, JR. GEORGE W. BAIN, JR.

No references cited.

